Sheet polarizers were developed to replace beam-splitter polarizers which are expensive, bulky and of limited size. The art of sheet polarizing material is well known dating from Edwin H. Land's invention of the H-sheet dichroic polarizer in 1938. Production of plastic polarizing materials in sheet form is a two step process. First, a suspension medium containing long chain molecules is stretched to align those long chain molecules. Second, polarizing dichroic molecules are added to the medium or included in the medium which attach themselves so as to be oriented along the aligned chain molecules. The light polarizing particles may also be dispersed in the medium and aligned by extruding, rolling or stretching the medium.
Although most sheet polarizing material marketed commercially has been the organic plastic material type, research has been performed on high performance glass polarizers for ophthalmic applications where high surface hardness and good scratch resistance characteristics are desired. Polarizing glasses have been prepared where ellipsoidal metallic particles are dissolved in the glass. The polarizing action is based on the fact that the ellipsoidal metal particles absorb light polarized along the long axis and transmit light polarized perpendicular to the long axis.
Three methods for making polarizing glass have been disclosed in recent patent literature. U.S. Pat. Nos. 3,540,793; 4,125,404; and 4,125,405 disclose a polarizing action in photochrome glasses containing silver halides which are darkened with actinic radiation in the range 350 nm to 410 nm and bleached with linearly polarized bleaching light.
U.S. Pat. Nos. 3,653,863 and 4,282,022 disclose the manufacture of highly polarizing glasses starting with glass which is phase separable or photochromic and contains a silver halide which is heat treated to form silver halide particles of the desired size. The glass is then subjected to a two step process. First, the glass is heated at an elevated temperature between the annealing point and the melting point (500.degree. C. to 600.degree. C.) followed by stretching, extruding or rolling the glass containing the silver halide particles to elongate them and orient the particles to an ellipsoidal shape. Second, the glass is subjected to irradiation by actinic radiation to produce silver metal on the surface of the silver halide particles. An improvement of the second step is disclosed in U.S. Pat. No. 4,304,584 where the extruded glass is heat treated in a reducing environment at temperatures below the annealing point of the glass in order to produce elongated metallic silver in the glass or on the silver halide particles in a surface layer of the glass at least ten microns thick. The process includes the making composite glass bodies where polarizing and photochromic glass layers are combined and laminated.
A further method for making polarizing glass is disclosed in U.S. Pat. No. 4,479,819 for the near infrared spectral region described as 700 nm to 3000 nm by improved glass drawing and high temperature reduction techniques. A further method is disclosed in U.S. Pat. No. 4,486,213 for the cladding of a core polarizing glass with a skin glass in order to achieve high aspect ratios for the elongated metal particles. U.S. Pat. No. 4,908,054 discloses methods for improving the contrast and the bandwidth of polarization action for the product described in U.S. Pat. No. 4,479,819.
A third class of polarizers are Hertzian polarizers which place metal wires on the surface of a transparent optical material. Prior to 1900, Heinrich Hertz demonstrated a method for polarizing radiation using an array of parallel reflective wires which were long compared to the wavelength of the radiation to be polarized, and the wires were separated by a distance much less than the wavelength to be polarized. The Hertzian polarizer is often configured as a grid of wires but can also be irregularly spaced wires which meet the polarization conditions. The Hertzian polarizer transmits the radiation with electrical vector perpendicular to the wires and reflects radiation with electrical vector parallel to the wires.
U.S. Pat. No. 3,046,839 discloses a method of manufacturing a Hertzian polarizer on the surface of an optical material by forming a diffraction grating on the surface. The diffraction grating consists of grooves and the groove tips are evaporatively coated with metal to form an array of metal filaments. U.S. Pat. No. 3,353,895 discloses a method of manufacturing a Hertzian polarizing material by forming metal filaments using an evaporative shadowing method. Evaporated metal is directed near the grazing angle toward a bumpy transparent material covered with protuberances. Metal filaments of a Hertzian polarizer are produced by forming filaments which lie along side the protuberances and are separated by the shadows cast by the protuberances.
U.S. Pat. Nos. 3,969,545 and 4,049,338 disclose a Hertzian polarizer having filaments of metal which are evaporatively deposited on smooth surfaces of transparent optical material. The metal elements of the Hertzian polarizer are silver whiskers grown on the surface by grazing angle vacuum deposition of silver.
Each of the three classes of sheet polarizers lack characteristics to produce a high performance polarizing material suitable for both the visible and near-infrared spectral region (400 nm to 3,000 nm). The plastic sheet polarizers have poor performance in the near-infrared spectral region and are easily damaged because of the softness of plastic. The Hertzian polarizers applied to optically transparent materials reflect rather than absorbs the unwanted polarization components of radiation which is particularly undesirable for ophthalmic and display applications. The Hertzian method, although successfully applied to the near-infrared spectral region, has not been effectively extended to the visible portion of the spectrum because of the difficulty of producing a uniform density of metal filaments spaced at separations much less than the wavelength of light. Finally the polarizing glass method is limited to glasses which are highly specialized compositions containing silver. Although the polarizing glasses under the trademark POLACOR are effective near-infrared polarizers, the original goal of manufacturing ophthalmic quality glass for use in quality and prescription sunglasses has not be achieved. This unmet goal is due to the complexity and difficulty of the shaping and heating of specialty glasses and a failure to control the shape and uniformity of the polarizing metal particles for the visible spectral region.
A need has thus arisen for a polarizer and method of manufacturing polarizing material having high performance for both the visible and near-infrared spectral regions.